ATI AT1505 Five-Channel Power Amplifier Review

Stereo Review May 1998
by Daniel Kumin, Start Laboratories

Amplifier Technologies Incorporated (ATI) is a relative newcomer to the U.S. audio scene, but the people behind it are not. The company's founders were associated with the SAE brand, widely known in the 1960s and 1970s for unpretentious, good, value-oriented preamps, power amps and other components. Although ATI's brand image appears to be more upscale than was SAE's, its products also seem designed and priced more for measurable performance and value than for esoteric appeal. This translates to a line of sensible and unusually robust power amplifiers (ATI's only product category so far) that are not only designed but also made in the U.S.

The latest example, the five-channel AT1505, is built with obvious care and quality. It lists for $1,695 and is rated to deliver 150 watts per channel into 8 ohms from 20 Hz to 20 KHz with less than 0.03 percent total harmonic distortion plus noise (THD+N), or 225 watts per channel into 4 ohms. The plain but attractive front panel has a great big on/off rocker switch and five small clipping LEDs, each of which lights when its associated channel has reached maximum power and glows more steadily as it further exceeds the clipping level. These lit up obediently on the test bench but barely glimmered during multichannel listening until the system achieved fairly ridiculous volume levels.

A very heavy, all-metal chassis holds the thick, subtly beveled front panel and the silver-gray steel top/side cover, which has a perforated cooling grille - a tasty touch. The whole business is assembled in solid, businesslike fashion with Allen-head screws and the very decent finish quality extends to the folded-steel chassis pan and rear panel.

The rear panel presents a gold-plated RCA input jack and a heavy-duty, five-way speaker output, also gold-plated, for each channel. Each output terminal is also equipped with two bayonet-screw fuseholders and two LEDs. The dual 3-ampere fuses guard each channel's positive and negative supplies and the associated LEDs are there simply to indicate when the adjacent fuse has been called into action. The idea is that if a channel suddenly sounds really bad or falls silent you can get a quick fix on the problem by glancing at the back panel.

There are no input-level trim controls or on-board bridging facilities. The remaining rear-panel connectors include an IEC socket for the removable power cord - ATI supplies a high-quality, heavier-than-usual cord and a computer-style DB-25 multichannel connector, still relatively rare on A/V components. The multipin connector accepts a five-channel (unbalanced) audio signal and a low-voltage DC on/off trigger from a single DB-25 cable.

Under the AT1505's hood, a very large toroidal power transformer is mounted on edge just inside the front panel. It has individual plus and minus secondary windings for each channel, a refinement that is said to reduce hum a bit and to decrease interaction among the channels. Behind the transformer are five identical power-amp modules, each of which has its own heatsink and contains all the other requisite circuitry for a single channel, including power-supply rectifiers and filter capacitors. The modules are designed for easy removal by a qualified technician. Construction is entirely first-rate, with lots of top-rate parts. ATI also provides a generous seven-year warranty covering parts and labor.

ATI says that the AT1505 design is fully complementary from input to output; that is, the circuit topology is symmetrically balanced. It's also DC-coupled throughout except for the first input stage, which can help preserve waveform integrity at near-overload levels. Each channel operates in Class A mode right up to the driver stage, where it controls an individual output stage composed of eight high-current bipolar transistors - all solidly high-end stuff. ATI further reports that thermal-protection sensors on each channel's heatsink are designed to shut down the amplifier if it overheats. That did not happen during my use or lab tests, not even after I let all the channels clip for several minutes, but shorting an output with a screwdriver (don't try this at home!) did pop the fuses.

On the test bench, the AT1505 easily surpassed its quite stringent power specifications, delivering just over 200 watts into a single-channel 8-ohm load and half again as much into 4 ohms. Rather impressively, the maximum 8-ohm output with all five channels driven simultaneously in phase was 157 watts per channel. This test included an AC-power-line sag (from idle) of about 7 volts in my studio, which is a tad less than I've seen with some other high-power multichannel amps. Dynamic headroom measured 1.6 and 2 dB at 8 and 4 ohms, but note that these figures really reflect clipping headroom more than true dynamic reserves, since the amplifier clipped at nearly the same levels regardless of whether the test signal was continuous or on/off tone bursts.

Noise and distortion were equally exemplary. Noise referred to 1 watt was just shy of -100 dB (about -121 dB referred to full output), which is more than admirable. Distortion (plus noise) remained below 0.02 percent at any power level from a few watts up to clipping. Frequency response was perfectly flat except for an inconsequential 0.14 dB droop at the very top of the band. Even more notable (and probably more important), the response measurements of all five channels were nearly identical, fitting within an utterly minuscule ±0.05-dB window - very impressive!

I auditioned the AT1505 in my home-theater reference system, which comprises a suite of moderate-sensitivity speakers: B&W Model 803 Series 2 and HTM speakers up front (left/right and center) and a pair of Citation 7.3 switchable dipole/bipole surround speakers. I did not employ a subwoofer. Source material included CDs, DVDs and laserdiscs.

With multichannel
program material, I was unable to induce audible
stress even at a very loud volume level.

After I chased down and eradicated a distracting ground-loop buzz, the ATI AT1505 was marvelously quiet, emitting just a faint, ';white' whoosh on the ear-to-the-tweeter test and virtually no buzz. Potentially more important in many rooms, the amplifier was entirely free of any perceptible mechanical buzzing from its power transformer, something that's far from universally true of big, high-power multichannel amps.

In terms of ease of use - well, what's to use? The AT1505 has no input-level controls to set, no special functions or features to learn and no displays to watch other than the clipping LEDs, which rarely light at normal volume levels. About the only eccentricity I can cite is that since it employs no output relays , the AT1505 continues to play for many seconds after its power is cut off, gradually fading into distortion as its capacitors discharge. I could not try out the automatic on/off feature as it requires a preamp or other controller equipped with a DB-25 connector, so there was nothing left to do but listen.

That I did, extensively and with great pleasure. With multichannel program material, whether movie soundtracks or music recordings (including a couple of 5.1-channel music discs), I was unable to induce audible stress at any reasonable to very loud volume level. While playing two-channel stereo recordings, the AT1505 produced ample clean volume from the B&W 803/2 speakers, although the clipping indicators did occasionally flash - we're talking seriously loud volume levels here.

Musically, the ATI-powered system sounded very neutral to my ear, with impressive dynamic reproduction and smooth but unrestricted treble. Stereo imaging was excellent and surround imaging with the best 5.1-channel recordings available to me, such as DVD Spectacular on Delos, was literally spine-tingling. Bottom-octave extension and overall sonic ease were almost as impressive. In terms of how it performs, the AT1505 concedes nothing to the other high-power multichannel amplifiers I've encountered.

ATI has produced a very fine amplifier that falls solidly in the middle of the current pack in terms of pricing. The AT1505's performance is indisputably top-shelf, which qualifies it as a paragon of value for home-theater, surround-sound power applications.

Measurements

All data reflect worst-case one-channel operation except as noted.